The present invention relates to a scanning transmission electron microscope (: STEM). More concretely, it relates to a control device and method for controlling an electron beam which has passed through a specimen.
The STEM is a device for visualizing specimen structure with a sub-nanometer high space resolution. A raster scanning of an electron beam which is converged down to a nanometer order is performed on a specimen whose film is thinned down to a few hundreds of nm. Moreover, a signal generated from an electron-beam irradiation area is detected, then being synchronized with the raster scanning. This synchronization allows the two-dimensional image to be formed. Examples of the signal generated from the electron-beam irradiation area are a transmitted electron beam, a secondary electron beam, and characteristic X-rays.
FIG. 2 is a basic configuration diagram of the general STEM. The direction parallel to an optical axis 20 of the lens-barvel is defined as the Z direction, and the plane perpendicular to the optical axis is defined as the XY plane. A primary electron beam emitted from an electron gun 1 is accelerated up to a few hundreds of kV, then being formed in shape by a first condenser lens 2 and a second condenser lens 3. Moreover, the primary electron beam passes through a condenser aperture 4 for restricting an aperture angle of the primary electron beam, then being focused on a specimen 21 by an upper objective lens 7-1. Although an objective lens is, physically, a single lens, the specimen 21 is set up in a gap of the objective lens. For this reason, the upper objective lens 7-1 and a lower objective lens 7-2 are assumed as the ray diagram. Here, the lens 7-1 allows focal point of the primary electron beam to be achieved on the specimen, and the lens 7-2 has a role of projecting the transmitted electron beam which has passed through the specimen 21. An irradiation lens system including these lenses permits the primary electron beam to be converged down into a sub-nm diameter on the specimen 21.
The transmitted electron beam which has passed through the specimen 21 is projected onto an electron-beam detection system by the lower objective lens 7-2 and a projection lens 9. A raster scanning of the electron beam is performed within the XY plane by a scanning coil 5. Furthermore, a control signal for the scanning coil 5 and an output signal from an electron detector 14 are synchronized with each other, thereby forming a STEM image within a computer, and displaying the STEM image on a monitor. The characteristic of the STEM is that changing the detection signal permits various specimen information to be imaged in an easy and convenient manner. For example, if high-angle scattered electrons are detected using an electron annular detector 12, a high-angle annular dark field (: HAADF) image can be acquired. If low-angle scattered electrons in proximity to the optical axis 20 are selected using an angle selection aperture 13, and if the low-angle scattered electrons selected are detected using the electron detector 14, a bright field (: BF) image can be acquired.
JP-A-2001-93459 has disclosed the following technology: Namely, according to this technology, in the device configuration for acquiring an electron energy loss spectrum (: EELS) image, a change in incident position of the transmitted electron beam relative to the electron detector is cancelled using a de-scanning coil. Here, this change will occur in accompaniment with a change in incident position of the primary electron beam relative to the specimen.
If the incident position of the primary electron beam relative to the specimen 21 is changed, the incident position of the transmitted electron beam relative to the electron detector changes.
Accordingly, when selecting electrons under a certain condition out of the transmitted electrons by an aperture or a slit, and detecting the selected electrons, the transmitted electron beam displaces relative to the electron detector. On account of this displacement, the condition of the transmitted electrons to be detected at the electron detector changes depending on the incident position of the primary electron beam. This condition change causes an artifact to occur in the STEM image. The use of the de-scanning coil makes it possible to suppress this artifact. In JP-A-2001-93459, however, no disclosure has been made concerning a concrete control method for the de-scanning coil.